1. Field of the Invention
The present invention relates to an operational amplifier, and more particularly, to an operational amplifier having an output section constituted by a push-pull circuit.
Further, the present invention also relates to a current detector circuit and an amplifier employing the current detector circuit, and more particularly, to a current detector circuit for selecting a smaller one from current values (two current values) of a current flowing through a detecting object (detecting device) and maintain the current flowing through the detecting object at a predetermined value and an amplifier employing the current detector circuit, capable of operating with a low source voltage.
2. Description of the Related Art
Recently, compact and low-power-consumption electronic apparatuses have been developed. Portable electronic devices are mostly driven by batteries. Homeuse and office-use electronic apparatuses driven by, for example, AC 100 (117, 200, or 220) V usually employ semiconductor integrated circuits (LSIs) operating with a power source voltage of 5 V.
The portable devices driven by battery, however, usually operate with a power source voltage of 3 V. The battery voltage decreases as the battery wares. LSIs in the portable devices must deal with such a fluctuation in the source voltage and a variety of source voltages ranging from 3 to 5 V. Namely, the LSIs must be stable for a wide range of source voltages.
To let an analog circuit operate for a wide range of source voltages, an operational amplifier plays an important role. An operational amplifier whose output section constituted by a push-pull circuit may be used for heavy load or a high-speed operation in an analog-digital mixed signal LSI consisting of CMOSs. In this case, the operational amplifier must be particularly stable. It is required to provide an operational amplifier that shows stable performance for a wide range of source voltages.
By the way, low-frequency amplifiers are classified into classes A, B, and AB. The A-class amplifiers involve a low signal distortion but large power consumption. The B-class amplifiers involve a large signal distortion but small power consumption. The AB-class amplifiers involve a small signal distortion and small power consumption.
Amplifiers for driving speakers must provide a large current at low distortion. For this purpose, the A-class amplifiers consume large power even when there are no signals, and the B-class amplifiers cause a problem of crossover. Accordingly, the AB-class amplifiers are usually employed to drive speakers.
To reduce power consumption, low-voltage power source are widely used. It is necessary, therefore, to develop amplifiers operating with a low-voltage power source.